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Lecture 4 Socket Programming

Lecture 4 Socket Programming. CPE 401 / 601 Computer Network Systems. slides are modified from Dave Hollinger. Network Application Programming Interface (API). Services that provide the interface between application and protocol software (often by the operating system). Application.

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Lecture 4 Socket Programming

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  1. Lecture 4Socket Programming CPE 401 / 601Computer Network Systems slides are modified from Dave Hollinger

  2. Network Application Programming Interface (API) • Services that provide the interface between application and protocol software (often by the operating system). Application Network API Protocol A Protocol B Protocol C Socket Programming

  3. Network API wish list • Generic Programming Interface • Support multiple communication protocol suites (families). • Address (endpoint) representation independence. • Provide special services for Client and Server? • Support for message oriented and connection oriented communication • Work with existing I/O services (when this makes sense) • Operating System independence Socket Programming

  4. TCP/IP • TCP/IP does not include an API definition. • There are a variety of APIs for use with TCP/IP: • Sockets • TLI, XTI • Winsock • MacTCP Socket Programming

  5. Functions needed: • Specify local and remote communication endpoints • Initiate a connection • Wait for incoming connection • Send and receive data • Terminate a connection gracefully • Error handling Socket Programming

  6. Berkeley Sockets • Generic: • support for multiple protocol families. • address representation independence • Uses existing I/O programming interface as much as possible. Socket Programming

  7. Socket • A socket is an abstract representation of a communication endpoint. • Sockets work with Unix I/O services just like files, pipes & FIFOs. • Sockets (obviously) have special needs: • establishing a connection • specifying communication endpoint addresses Socket Programming

  8. Unix Descriptor Table Descriptor Table Data structure for file 0 0 1 Data structure for file 1 2 3 Data structure for file 2 4 Socket Programming

  9. Socket Descriptor Data Structure Descriptor Table Family: PF_INET Service: SOCK_STREAM Local IP: 111.22.3.4 Remote IP: 123.45.6.78 Local Port: 2249 Remote Port: 3726 0 1 2 3 4 Socket Programming

  10. Creating a Socket int socket(int family,int type,int proto); • family specifies the protocol family (PF_INET for TCP/IP). • type specifies the type of service (SOCK_STREAM, SOCK_DGRAM). • protocol specifies the specific protocol (usually 0, which means the default). Socket Programming

  11. socket() • The socket() system call returns a socket descriptor (small integer) or -1 on error. • socket() allocates resources needed for a communication endpoint • but it does not deal with endpoint addressing. Socket Programming

  12. Specifying an Endpoint Address • Sockets API is generic. • There must be a generic way to specify endpoint addresses. • TCP/IP requires an IP address and a port number for each endpoint address. • Other protocol suites (families) may use other schemes. Socket Programming

  13. Necessary Background Information: POSIX data types int8_tsigned 8bit int uint8_tunsigned 8 bit int int16_tsigned 16 bit int uint16_t unsigned 16 bit int int32_tsigned 32 bit int uint32_t unsigned 32 bit int u_char, u_short, u_int, u_long Obsolete Socket Programming

  14. More POSIX data types sa_family_t address family socklen_tlength of struct in_addr_tIPv4 address in_port_tIP port number Socket Programming

  15. Generic socket addresses Used by kernel struct sockaddr { uint8_t sa_len; sa_family_t sa_family; char sa_data[14]; }; • sa_familyspecifies the address type. • sa_dataspecifies the address value. Socket Programming

  16. sockaddr • An address that will allow me to use sockets to communicate with my family. • address type AF_FAMILY • address values:Daughter 1 Wife 2 Mom 3 Dad 4 Sister 5 Brother 6 Socket Programming

  17. AF_FAMILY • Initializing a sockaddr structure to point to Daughter: struct sockaddr mary; mary.sa_family = AF_FAMILY; mary.sa_data[0] = 1; Socket Programming

  18. AF_INET • For AF_FAMILY we only needed 1 byte to specify the address. • For AF_INET we need: • 16 bit port number • 32 bit IP address IPv4 only! Socket Programming

  19. struct sockaddr_in (IPv4) Length of structure (16) struct sockaddr_in { uint8_t sin_len; sa_family_t sin_family; in_port_t sin_port; struct in_addr sin_addr; char sin_zero[8]; }; struct in_addr { in_addr_t s_addr; }; AF_INET 16 bit Port number Make structure 16 bytes 32 bit IPv4 address Socket Programming

  20. struct sockaddr_in (IPv6) Length of structure (28) struct sockaddr_in6 { uint8_t sin6_len; sa_family_t sin6_family; in_port_t sin6_port; uint32_t sin6_flowinfo; struct in6_addr sin6_addr; uint32_t sin6_scope_id; }; struct in6_addr { uint8_t s6_addr[16]; }; AF_INET6 Port number Flow label Scope of address 128 bit IPv6 address Socket Programming

  21. Network Byte Order • All values stored in a sockaddr_inmust be in network byte order. • sin_porta TCP/IP port number. • sin_addran IP address. !!! Common Mistake !!! Socket Programming

  22. Network Byte Order Functions ‘h’ : host byte order ‘n’ : network byte order ‘s’ : short (16bit) ‘l’ : long (32bit) uint16_t htons(uint16_t); uint16_t ntohs(uint_16_t); uint32_t htonl(uint32_t); uint32_t ntohl(uint32_t); Socket Programming

  23. TCP/IP Addresses • We don’t need to deal with sockaddr structures since we will only deal with a real protocol family. • We can use sockaddr_instructures. • BUT: The C functions that make up the sockets API expect structures of type sockaddr. Socket Programming

  24. sockaddr_in6 sockaddr sockaddr_in length AF_INET6 sa_len length port sa_family AF_INET Flow-label sa_data port addr addr Scope ID zero variable 16 bytes 28 bytes Socket Programming

  25. Assigning an address to a socket • The bind() system call is used to assign an address to an existing socket. int bind( int sockfd, const struct sockaddr *myaddr, int addrlen); • bind returns 0 if successful or -1 on error. const! Socket Programming

  26. bind() • calling bind()assigns the address specified by the sockaddr structure to the socket descriptor. • You can give bind()a sockaddr_instructure: bind( mysock, (struct sockaddr*) &myaddr, sizeof(myaddr) ); Socket Programming

  27. bind() Example int mysock,err; struct sockaddr_in myaddr; mysock = socket(PF_INET,SOCK_STREAM,0); myaddr.sin_family = AF_INET; myaddr.sin_port = htons( portnum ); myaddr.sin_addr = htonl( ipaddress); err=bind(mysock, (sockaddr *) &myaddr, sizeof(myaddr)); Socket Programming

  28. Uses for bind() • There are a number of uses for bind(): • Server would like to bind to a well known address (port number). • Client can bind to a specific port. • Client can ask the O.S. to assign any available port number. Socket Programming

  29. Port schmort - who cares ? • Clients typically don’t care what port they are assigned. • When you call bind you can tell it to assign you any available port: myaddr.port = htons(0); Socket Programming

  30. What is my IP address ? • How can you find out what your IP address is so you can tell bind() ? • There is no realistic way for you to know the right IP address to give bind() • what if the computer has multiple network interfaces? • specify the IP address as: INADDR_ANY, this tells the OS to take care of things. Socket Programming

  31. IPv4 Address Conversion int inet_aton( char *, struct in_addr *); • Convert ASCII dotted-decimal IP address to network byte ordered 32 bit value. • Returns 1 on success, 0 on failure. char *inet_ntoa(struct in_addr); • Convert network byte ordered value to ASCII dotted-decimal (a string). Socket Programming

  32. IPv4 & IPv6 Address Conversion int inet_pton( int, const char *, void *); • (family, string_ptr, address_ptr) • Convert IP address string to network byte ordered 32 or 128 bit value. • Returns 1 on success, -1 on failure, 0 on invalid input char *inet_ntop(int,const void*,char*,size_t); • (family, address_ptr, string_ptr, length) • Convert network byte ordered value to IP address string • x:x:x:x:x:x:x:x or x:x:x:x:x:x:a.b.c.d Socket Programming

  33. Other socket system calls • Connection-oriented (TCP) • connect() • listen() • accept() • General Use • read() • write() • close() • Connectionless (UDP) • send() • recv() Socket Programming

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